Self-encapsulated silver alloys for interconnects

ABSTRACT

Alloys of silver and an alloying element that diffuses to the surface of the high conductivity metal and is oxidizable to form an alloying element oxide such as beryllium are provided along with electronic structures employing the alloys and methods of fabrication.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a divisional application of U.S. Ser. No. 10/697,014filed Oct. 31, 2003 and now U.S. Pat. No. 7,189,292.

TECHNICAL FIELD

The present invention relates to interconnects, that are especiallyuseful for back end of the line (BEOL) metallic conductors inmicroelectronics devices such as memory and logic devices. A particularembodiment of the present invention relates to a silver-beryllium alloythat is self-encapsulated in beryllium oxide protecting it from theeffects of the ambient thereby making it possible to stand alone withouta need for barrier layers and capping layers. The present inventionenables the use of porous dielectrics and air dielectric (air bridges).

BACKGROUND OF THE INVENTION

With transistor sizes becoming smaller and smaller and switching speedsfaster and faster, transmission delay through interconnections betweenactive devices is a major cause of concern. The delay depends on theresistance-capacitance and is referred to as RC delay. The resistancepart of the signal delay is due to the resistivity of the metallicinterconnect while the capacitance depends on the dielectric constant ofthe medium applied between the conductor lines. In order to reduce thedielectric constant SiO₂ is being replaced by various low-k materials.In an effort to further reduce the dielectric constant these newmaterials are used in a porous form and since the dielectric constant ofair is 1, ultimately some designs have part of the dielectric replacedby air (air bridges). This brings with it the increased need to protectthe metal conductor from the ambient.

A few years ago Al—Cu alloy interconnects were replaced by copperinterconnects due to the lower restitivity of copper, that is 1.68 microOhm-cm versus the 2.65 micro Ohm-cm resistivity of unalloyed aluminum.Copper has however a very great propensity for oxidation and the Cu₂Othat forms up to 300° is not a barrier to further oxidation having anopen crystal structure.

Consequently copper is protected by barrier layers and capping layers.As the line width of interconnects decreases, the thickness of thelayers protecting the copper lines in all directions becomes asignificant fraction of the copper line width. Since these protectivelayers are neither good conductors nor low-k dielectrics, some of thegain in using a lower resistivity conductor and low-k dielectrics islost.

Also, the National Technology Roadmap for Semiconductors predicts thatfor the 100 nm device dimension node, a material with a dielectricconstant of about 2.0 will be needed to reduce the RC delay associatedwith BEOL interconnects. The most promising approach to developing suchmaterials is to introduce porosity into a dense dielectric material. Oneof the problems of integrating these dielectrics into copper or silverbased dual damascene schemes is that exposure to ambient conditions evenat a relatively low temperature tends to cause oxide and sulphideformation impairing the conductivity of these originally highconductivity metals.

SUMMARY OF INVENTION

The present invention addresses problems of oxidation and/orsulphidation of silver. In particular, according to the presentinvention the oxidation and/or sulphidation resistance of silver isimproved by alloying with certain types of alloying elements.

In particular, the present invention relates to an alloy of silver andan alloying element wherein the alloying element does not form a solidsolution with silver or an intermediate phase under 700° C. and diffusesto the surface of the silver at temperatures of 400° C. or below; and isoxidizable to form an alloying element oxide having a conductivity ofless than 10⁻⁵ reciprocal Ohm-cm.

Another aspect of the present invention relates to an interconnectstructure comprising the above disclosed alloy; and a layer of thealloying element oxide of about 2 to about 10 nanometers thick locatedon the alloy.

A further aspect of the present invention relates to an electronicstructure comprising a dielectric layer having a substantially planarupper surface and having a pattern of recess therein, and the abovedisclosed alloy being located in recesses.

The present invention also relates to a method for fabricating aninterconnect structure which comprises providing an alloy as disclosedabove; and selectively oxidizing the alloying element by annealing attemperature of about 250° C. to about 500° C. in an oxidizing atmospherehaving an oxidizing agent at a partial pressure of about 10⁻⁸ to about 1Torr forming a layer of alloying element oxide on the alloy.

A still further aspect of the present invention relates to a process forfabricating an interconnect structure on an electronic device whichcomprises: forming a patterned resist layer on a substrate havinginsulating regions and conductive regions, depositing the abovedisclosed alloy; and removing the patterned resist.

A further aspect of the present invention relates to the process forfabricating an interconnect structure or an electronic device whichcomprises:

forming an insulating material on a substrate;

lithographically defining and forming recesses for lines and/or vias inthe insulating material in which interconnection conductor material willbe deposited;

depositing an interconnection conductor material comprising the abovedisclosed alloy; and planarizing the resulting structure to provideelectrical isolation of individual lines and/or vias.

The present invention further relates to a process for fabricating aninterconnect structure on an electronic device which comprises:

depositing an insulating material on a substrate,

lithographically defining and forming lines and/or vias in whichinterconnection conductor material will be deposited;

forming a patterned resist layer on the insulating material; depositinga conductor material comprising an alloy as disclosed above;

and removing the patterned resist.

Also, the present invention relates to fabricating an interconnectstructure or an electronic device which comprises:

depositing a blanket layer of the above disclosed alloy on a substratehaving insulating regions and conductive regions;

forming a patterned resist layer on the blanket layer,

removing the conductor material where not covered by the patternedresist and removing the patterned resist.

In addition, according to the present invention, instead of depositingan alloy as disclosed above, structures can be fabricated, for variousintegration schemes, by depositing in the patterned structure arelatively thin layer of beryllium as a liner barrier followed bydepositing silver to the desired thickness. The structure is subjectedto an anneal, e.g at temperatue disclosed above. BeO is produced onthree sides of the silver but not on the top of the silver in thisfabrication technique. In the event, the silver is to be deposited byelectroless plating or electroplating instead of PVD, a seed layer ofsilver is deposited between the beryllium and silver deposit. The seedlayer is typically thick enough to form a continuous or substantiallycontinuous layer.

Other objectives and advantages of the present invention will becomereadily apparent to those skilled in this art from the followingdetailed description, wherein it is shown and described only thepreferred embodiments of the invention simply by way of illustration ofthe best mode contemplated of carrying out the invention. As will berealized, the invention is capable of other and different embodiments,and its several details are capable of modifications in various obviousrespects, without departing from the invention. Accordingly, thedescription is to be regarded as illustrative in nature and not asrestrictive.

SUMMARY OF DRAWINGS

FIGS. 1A and 1B are Ag—Be equilibrium phase diagrams (atomic percent—1Aand weight percent—1B).

FIGS. 2A-2E illustrate an air-bridge structure incorporating a conductoraccording to the present invention.

BEST AND VARIOUS MODES FOR CARRYING OUT INVENTION

In order to facilitate an understanding of the present inventionreference is made to the drawings. FIG. 2A illustrates a structure forfabricating a dual damascene structure. In FIG. 2A, a via 16 is providedin an isolation layer 14 on a semiconductor substrate 12 such assilicon, silicon-germanium alloys, and silicon carbide or galliumarsenide.

Exemplary isolation or dielectric 14 include silicon dioxide (SiO2),phosphosilicate glass (PSG), boron doped PSG (BDPSG) ortetraethylorthosilicate (TEOS). In addition, the dielectric can includelow dielectric constant material such as CVD carbon-doped oxide, porousCVD carbon-doped oxide, porous and non-porous spin-on organo silicates,porous and non-porous organic spin-on polymers.

In FIG. 2B, an alloy layer is deposited such as by physical vapordeposition including electron beam evaporation or magnetron sputtering.As an example, the alloy is sputtered from a target defined herein belowin an argon atmosphere of about 2 to about 15 milliTorr, more typicallyabout 4 to about 10 mTorr and applying a power of about 50 to about 600watts, and typically about 200 to about 400 watts.

The alloy includes silver. According to the present invention, thealloying constituent employed does not form a solid solution with silveror an intermediate phase with the temperatures below 700° C. and thusdoes not impair to an undesired extent the conductivity of the metal.The alloying element is segregated to the surface of the metal bydiffusion at temperatures of 400° C. or below and is selectivelyoxidized. The metal does not oxidize because the metal is encapsulatedin the oxide of the alloying element. This oxide has a high resistivityand therefore it does not permit electron transport that is necessaryfor oxide growth. Consequently, a self-limiting oxide will be createdthat is relatively thin, typically in the order of about 2 to about 10nanometers. The oxide typically has a relatively low dielectric constantso as not to cause significant capacitance increase and also typicallyhas excellent heat conductivity.

The preferred alloying element is beryllium. Other alloying elements areAl and Si. Typical alloys according to the present invention aresilver-beryllium alloys typically containing about 0.2% by weight toabout 5% by weight of beryllium, more typically containing about 0.2% byweight to about 3% by weight of beryllium and even more typically about0.2% by weight to about 2% by weight of beryllium.

FIGS. 1A and 1B give the silver beryllium equilibrium phase diagramshowing the absence of mutual solid solubility of silver and berylliumas well as intermediate phase formation under 760° C. The figures showthe phase diagram both for atomic percent (1A) and for weightpercent(1B). Due to the large difference between the density ofberyllium (a light metal) and silver (a heavy metal) small weightpercentages of beryllium represent large atomic percents.

The alloy is for example sputtered from a Ag—Be target, or may beco-sputtered from separate silver and beryllium targets. On top of thealloy layer a thin silver layer may be deposited to protect the alloyfrom exposure to an electrolyte.

The silver-beryllium layer acts as a seed layer for subsequent platingof silver. The silver can be electroplated or electroless plated to fillthe vias. Silver is normally electroplated using potassium cyanidesolutions, which exhibit the highest plating current densities.Electroless silver is plated from a two-solution silvering system ofNaOH and ammoniacal Ag NO₃ mixed with a glucose solution containing anamine group. See FIG. 2C.

The filled structure can then be planarized such as by chemicalmechanical planarzation or electropolishing. This process can berepeated for several levels. Some of the low-k dielectric is removed byetching back to create air bridges 18 as illustrated in FIG. 2D.

Selective oxidation of the beryllium can be carried out at temperatureof about 250 to about 500° C. and in an atmosphere of low partialpressure of an oxidizing agent such oxygen or water vapor such as about10⁻⁸ to about 1 Torr. The time of the heat treatment is typically about10 to about 30 minutes depending on temperature. During the heattreatment, the beryllium diffuses to the surface of the silver forming athin and compact film on the surface. The exposure to the oxygencontaining gas produces a BeO film 19 on the surface, which drives thefurther diffusion of beryllium to the surface and protects theunderlying silver against oxygen or sulphur containing gases.Thermodynamic considerations favor the formation of BeO instead of Ag₂Osince the heats of formation are 286 and 14 kcal/mole oxygen,respectively. See FIG. 2E. It is important to note that BeO forms onlyat the surface of silver at interfaces with dielectric materials but notat the interface with a metallic via. It is also important to note, thatAg₂O decomposes to silver and oxygen at temperatures of about 190°Centrigrade.

Other fabrication techniques in addition to creating damascene structurecan, of course, be employed pursuant to the present invention. Suchtechniques include fabricating conductor elements.

The process for through-mask plating on a planar base includes providingan insulating layer on a substrate followed by a patterned resist layer.The alloy of the present invention is plated through the patternedresist. The resist is then removed.

The process for through-mask plating on an excavated base comprisesforming a via or channel in an insulating layer on a substrate. Apatterned resist is formed over the insulating layer. The alloy of thepresent invention is plated through the mask or resist into the vias.The mask or resist is then removed.

The process for blanket plating comprises blanket plating an alloy ofthe present invention over an insulating layer on substrate . A layer ofresist is formed over the blanket layer and lithographically patterned.The blanket layer is then patterned by etching or removing by otherprocesses where not protected by resist. The resist is then removed.

The following non-limiting examples are presented to further illustratethe present invention.

EXAMPLE 1

The trenches of a patterned low-k dielectric BEOL structure are filledby physical vapor deposition with 0.2% by weight Be—Ag alloy andplanarized with CMP. This process is repeated for several levels. Someof the low-k dielectrics may be etched back to create air bridges. Aftereach level is completed or after a plurality of or all levels arecompleted, an annealing takes place at 250° C. for 20 minutes in 6×10⁻⁸Torr partial pressure of oxygen. During this treatment the berylliumdiffuses to the surface of the multilevel conductor and oxidizes to BeO,which encapsulates the silver metal.

EXAMPLE 2

The trenches of a patterned low-k dielectric BEOL structure are filledby chemical vapor deposition with a 0.3% by weight Be—Ag alloy, and theprocess continues as in Example 1.

EXAMPLE 3

The trenches of a patterned porous low-k dielectric BEOL structure arelined with a 3% by weight Be—Ag alloy by physical vapor deposition.Subsequently the trenches are filled with pure silver deposited also byphysical vapor deposition and planarized with CMP. The process iscontinued as in Example 1.

EXAMPLE 4

The trenches of a patterned low-k dielectric BEOL structure are linedwith a 2.0% by weight Be—Ag alloy by physical vapor deposition, whichserves as a seed layer for the subsequent electroplating. Pure silver isthen electroplated from a known cyanide silver plating bath at therelevant current densities. The filled structure is planarized with CMPand the processes repeated on several levels. After each level iscompleted or after more than one level is completed, an annealing takesplace at 350° Centigrade for 10 minutes at 1×10⁻⁸ Torr partial pressureof oxygen. During this treatment the beryllium diffuses from the linerto the surfaces of the conductor and oxidizes to BeO which encapsulatesthe silver metal. This example is depicted in FIG. 2.

EXAMPLE 5

Example 4 is repeated except that silver is deposited on the liner/seedlayer with electroless plating. The electroless plating bath is an AgNO₃based bath.

EXAMPLE 6

Example 5 is repeated, except that over the Ag—Be liner a thin layer ofpure silver is deposited also by physical vapor deposition to protectthe beryllium from the electrolyte.

EXAMPLE 7

The trenches of the dielectric are lined with about 0.5 to about 3nanometers of pure beryllium deposited by atomic layer deposition orother suitable means followed by the deposition of a silver seed layerfollowed by filling the trenches by the electroplating or electrolessplating of silver. Afterwards the structure is annealed at temperaturesof about 250° C. to about 500° C. The annealing allows for the formationof a thin BeO barrier layer by gettering oxygen from the dielectric. BeOis formed on three sides of the silver but not on the top of the silver.In the following CMP, the BeO may be left remaining between theconductor lines since it is an insulator with a dielectric constant of6.

All publications and patent applications cited in this specification areherein incorporated by reference, and for any and all purposes, as ifeach individual publication or patent application were specifically andindividually indicated to be incorporated by reference.

The foregoing description of the invention illustrates and describes thepresent invention. Additionally, the disclosure shows and describes onlythe preferred embodiments of the invention but, as mentioned above, itis to be understood that the invention is capable of use in variousother combinations, modifications, and environments and is capable ofchanges or modifications within the scope of the invention concept asexpressed herein, commensurate with the above teachings and/or skill orknowledge of the relevant art. The embodiments described hereinabove arefurther intended to explain best modes known of practicing the inventionand to enable others skilled in the art to utilize the invention insuch, or other, embodiments and with the various modifications requiredby the particular applications or uses of the invention. Accordingly,the description is not intended to limit the invention to the formdisclosed herein. Also, it is intended that the appended claims beconstrued to include alternative embodiments.

1. A process for fabricating an interconnect structure on an electronicdevice which comprises: forming an insulating material on a substrate,wherein said insulating material comprises at least one member selectedfrom the group consisting of silicon dioxide, phosphosilicate glass,boron doped PSG, tetraethylorthosilicate and a low-k dielectric materialand said low-k dielectric material comprises at least one memberselected from the group consisting of CVD porous carbon-doped oxide,non-porous carbon-doped oxide, porous spin-on organo silicates,non-porous spin-on organo silicates, porous spin-on organic polymers andnon-porous spin-on organic polymers; lithographically defining andforming recesses for lines and/or via in said insulating material inwhich interconnection conductor material will be deposited; depositingan interconnection conductor material comprising an alloy of silver andberyllium, wherein the amount of beryllium is about 0.2 to about 5% byweight; wherein the beryllium is deposited in recesses and the silver isdeposited above the beryllium; and providing a silver seed layer betweensaid beryllium and silver; selectively oxidizing the beryllium byannealing at temperatures of about 250° C. to about 500° C. in anoxidizing atmosphere containing an oxidizing agent having a partialpressure of about 10⁻⁸ to about 1 Torr wherein the oxidizing agentcomprises oxygen or water vapor thereby forming a layer of berylliumoxide on the alloy; and planarizing the resulting structure to provideelectrical isolation of individual lines and/or vias.